Design of Coalescing Media
for Oil in Water

Oil in Water Coalescing Media – How It Works

An oil water separator, or OWS, contains coalescing media. This media provides a suitable surface for oil droplets to meet and grow, or coalesce, into larger droplets. As oil droplets grow in size the buoyancy of the droplets increases. The droplets rise towards the surface of the water due to the fact that the specific gravity of oil is less than the specific gravity of water.

In this way the oil will form a layer that can be separated from the water by skimming action before the water is reused or discharged. Any heavy solids present in the water being treated, or sludge, in theory should fall into the sludge compartment of the OWS unit. As oil droplets coalesce into larger droplets, the buoyancy of the droplets increases. This is reflected in the known rise time for a given size of oil droplet. The more efficient the coalescing action of the media, the larger the oil droplets become. Larger oil droplets result in reduced rise time:

Time Needed for Oil Droplet (0.85 Sp. Gr.)
in Water to Rise 3 inches
Droplet Size (microns) Rise Time
300 12 sec
150 42 sec
60 4 min 12 sec
30 17 min 24 sec
15 1 hour 8 min 54 sec
5 10 hour 2 min 9 sec

Stoke's Law

The importance of oil droplet size is obvious in the above table. This importance is also noted in Stoke's Law that defines the terminal rise velocity of a given size.

V = (2gr2) (Δd) / 9μ
g = acceleration of gravity constant
r = oil droplet radius
Δd = difference in Sp. Gr. of water and oil
μ = viscosity of water

Note: V increases in proportion to the square of droplet radius!

Problems with traditional coalescing media

Traditional oil water coalescing media have been variations of corrugated, inclined plates and mesh pads. By definition half of the surface of this type of media must always be aligned upward — which is the wrong direction for support of coalescing action. To attempt to overcome this problem by adding more specific surface, plate separation can be made more narrow. This solution only solves the problem partially as the ratio of incorrect to correctly oriented surface remains 50/50.

Possible plugging and fouling of the coalescing media is also supported by the ~ 45º angle of the inclined plate design and exacerbated in mesh pad design. For example, plugging of the existing oil water coalescing media was the major problem in the oil water separation unit at a car wash in Finland (see Case Study). Heavy particles could not fall into the sludge compartment due to interference from the inclined plates. HD Q-PAC®, made of oleophilic polypropylene, with all rounded surfaces at a 90º angle vs. the direction of water flow and many slender rods pointing down towards the sludge compartment, offers a solution to the maintenance problems that, until now, have been accepted in the oil water separation industry.

See more Advantages of HD Q-PAC